The treatment of wastewater often involves the removal of particulate contaminants by such well-known technologies as clarification, dissolved air flotation (DAF), and induced air flotation (IAF). These technologies have been used in wastewater treatment systems for many years. Clarification is a gravity separation process wherein the wastewater is passed through a basin or tank and the suspended solids are permitted to settle to the bottom for collection and disposal. The clarification process assumes the suspended solids have a density greater than that of the wastewater being processed.
In dissolved air flotation (DAF), the wastewater or a portion thereof is saturated with air under pressure, for example at a pressure around 60 psig, and then introduced through a submerged inlet into an open treatment reservoir. At ambient pressure, the solubility of air in water is diminished, so that an excess of air is released from the wastewater in the form of very small bubbles. As the air bubbles move toward the surface of the wastewater, they contact and attach to particulate contaminants to form a particle-air aggregate having a lower density than the density of the particle alone. Since the vertical velocity is directly proportional to the difference between the density of water and the density of the particle, a decrease in the density of the particle (because it is now attached to air) results in an increase in the vertical velocity of the particle in water. Consequently, the particle aggregate moves toward the surface of the water more rapidly. The contaminant particles are skimmed from the surface of the wastewater to produce a treated or decontaminated wastewater containing a reducing amount of particulate contaminants.
In induced air flotation (IAF), air is induced into the wastewater by mechanical means. The induced air forms bubbles, which rise toward the surface of the wastewater. As the bubbles move upward, they contact and become attached to particulate contaminants to promote flotation of the contaminants, which are then skimmed from the surface. As in the DAF process, the bubbles function to increase the vertical velocity of the particulate contaminants in accordance with Stokes' law.
The treatment of wastewater also commonly involves chemical treatment of the wastewater to assist in removing pollutants or contaminants. Chemicals are often used for pH adjustment and for reaction with specific pollutants. Furthermore, certain treatment chemicals promote the formation of aggregates, of either suspended solids or oil and grease, which have larger diameters than the individual particulates. These aggregates settle out or rise to the surface more rapidly. According to Stokes' equation, the vertical velocity of a particle in water is directly proportional to the square of the particle's diameter; thus a two-fold increase of the particles diameter will produce a four-fold increase in the vertical velocity. For example, it is expected that a particular contaminant having a diameter of 2D will move either to the surface or to the bottom of a stream of water in about one fourth the time required for a particular contaminant otherwise identical having a diameter D. Accordingly, the use of such chemicals may be used advantageously in clarification and flotation processes to promote the aggregation of smaller particles into large particles, increase the separation velocities (either settling or rising), and thereby improve the separation rates and efficiencies. The result of the use of these chemical treatments with clarification and flotation processes is an effluent decontaminated wastewater having a reduced level of suspended solids and hydrocarbons. Many such chemical treatments are well known.
Wastewater treatment systems and methods incorporating such technologies are disclosed in my earlier patents:                (1) U.S. Pat. No. 5,173,184 issued Dec. 22, 1992        (2) U.S. Pat. No. 5,288,737 issued Feb. 22, 1994        (3) U.S. Pat. No. 5,423,981 issued Jun. 13, 1995        (4) U.S. Pat. No. 5,534,159 issued Jul. 9, 1996        (5) U.S. Pat. No. 5,755,973 issued May 26, 1998        
FIG. 1 schematically illustrates a prior art wastewater treatment system based on the technology of the above patents and similar to those that have been manufactured by Hydro-Modular Systems Inc., of Oklahoma City, Okla. This system employs gravity separation and dissolved air flotation assisted by chemical treatment.
The system receives industrial wastewater from a plant and directs the dirty wastewater to a wastewater storage unit 100. The wastewater often contains suspended solids and hydrocarbons such as oil and grease. The storage unit 100 shown in FIG. 1 has conical pits that collect some of the suspended solids or oil and grease as sludge. The settled out sludge stays at the bottom of the wastewater storage unit 100. The pH of the wastewater is checked in the wastewater storage unit and is adjusted by addition of acid, for this particular wastewater, or caustic. The dirty wastewater is pumped by pump 101 from the storage unit 100 to an equalization tank 102. The equalization tank may help balance the amount of wastewater flowing through the system. It may operate to adjust the amount of flow out when the flow in is uneven. Chemicals are added in the equalization tank 102 to the wastewater to assist in the formation of larger particles or aggregates of the particulate matter that is suspected in the wastewater. A coagulant is supplied to the equalization tank from a drum 103 by a pump 151 and a coagulant aid is supplied from a drum 104 by pump 150. A mixer 105, attached to the top of the tank 102, mixes the chemicals with the dirty wastewater received from the wastewater storage unit 100.
The mixed dirty wastewater and coagulant chemicals are pumped by pump 106 from the equalization tank through mixing tubes 107 to a mixing, flotation, and discharge unit 108. A flocculent from a flocculent container 109 is added to the dirty wastewater and coagulant chemicals that pass through the mixing tube 107. Pressurized air is supplied from a source 110 and is injected into the dirty wastewater, coagulant, flocculent mix that is applied to the input of the unit 108. The wastewater treatment system is primarily a dissolved air flotation system. The dirty wastewater with chemicals and air enters the mixing area 111 of the unit 108. Internal baffles near the input end of the unit 108 form the mixing area 111 of the unit where the dirty wastewater with chemicals and air is injected. The air present in the dirty water is in the form of large bubbles, which are reduced in size by recycled wastewater applied to a nozzle (not shown) inside the mixing area. This recycled wastewater flows through line 112 and is injected with air from a source 113 before being sprayed into the mixing area. Sometimes, the recycled wastewater from the output of unit 108 is applied to the inlet of the mixing area 111.
Baffles are placed in the output end 114 and form a central quiescent zone, which is the flotation area of the unit 108. The operation of the baffles is shown in FIG. 3 of U.S. Pat. No. 5,755,973 (the '973 patent). The internal construction of a typical mixing, flotation and discharge unit of the prior art is shown in FIG. 3 of the '973 patent. The baffles in the discharge zone determine the level of the wastewater that is processed in the unit.
The suspended particles that are more dense than the wastewater settle in the bottom of the unit in the cones 115, 116, and 117. The lighter particles that are less dense than the water float to the top in the quiescent zone with the movement of the less dense particles to the surface being assisted by the dissolved air in the wastewater. These floating particles typically create a foam or floc blanket on the top of the wastewater in the quiescent zone. A rake, as shown in FIG. 3 of the '973 patent, is employed to sweep or rake off this foam into a sludge receptacle 118 mounted on the side of the unit 108. The sludge goes to the bottom of receptacle 118 and cones 115-117 and is removed by being pumped from the cones and receptacle to a sludge storage tank 120.
A pump 121 is provided in the line between the unit 108 and the storage tank 120 to remove the sludge from the unit 108 for storage in tank 120. Sludge is removed from the wastewater storage tank 100 by a pump 122 and is pumped to the storage tank 120. The sludge in tank 120 is typically mixed with some material to help in the de-watering process and transported from the storage tank 120 to a press 123 by a pump 124. The sludge is de-watered in the press 123 and a cake is formed. The caked sludge is taken out of the press 123 for proper disposal.
One failing of the mixing, flotation, and discharge unit 108 is that it is custom made based on each customer's needs and specification. The unit 108 is often specifically designed and made to have a particular size that depends upon the volumetric flow and contamination of the dirty wastewater feeding the system. Consequently, the cost of providing the unit 108 is relatively high. The unit 108 may also be inflexible to changes in the volumetric flow, such as if the industrial plant expands and begins producing more wastewater, and a new custom designed and made unit 108 may need to be installed when the increased volumetric flow sufficiently degrades the performance of the existing unit. In a good case scenario the existing layout of the wastewater system will permit such an addition, otherwise more extensive and more costly system revamping may be needed.